Best option in my opinion would be the sim card from Tschibo (They sell usually coffee). It's 10Euro for 500MB and 20Euro for 5GB per month. I just don't know how I would activate the card without a laptop and the sim is a regular size. So it would need to be cut down to fit in the iPad.

The store will only be able to activate via snail mail.
So you would want to activate via internet. I think some Burger Kings and all of Starbucks locations should have free wifi.
You'd probably also need a valid address for registration and maybe some money to load up the card initially.

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